Outdoor fan and indoor blower controller for heating, ventilation and air conditioning system and method of operation thereof

ABSTRACT

An HVAC controller, a method of operating an HVAC controller and an HVAC system employing the controller or the method. In one embodiment, the HVAC controller includes: (1) a processor couplable to at least two refrigerant pressure sensors via separate data paths to receive input signals therefrom and further couplable to a compressor stage and a condenser fan to provide output signals thereto, and (2) memory coupled to the processor and storing a software program having program instructions capable of causing the processor to command the compressor stage or the condenser fan to turn on irrespective of a state of an input signal generated by either of the at least two refrigerant pressure sensors, and generate an error message at least partially depending upon whether or not a high pressure shutdown occurs after the processor commands the compressor stage or the fan to turn on.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/694,392 entitled “Outdoor Fan and Indoor Blower Controller forHeating, Ventilation and Air Conditioning System and Method of OperationThereof”, filed on Jan. 27, 2010 which claims the benefit of U.S.Provisional Application Ser. No. 61/180,405, filed by Mark Beste, etal., on May 21, 2009, entitled “Comprehensive HVAC Control System,”commonly assigned with this application and incorporated herein byreference.

TECHNICAL FIELD

This application is directed, in general, to heating, ventilation andair conditioning (HVAC) systems and, more specifically, to an outdoorfan and indoor blower controller for an HVAC system and method ofoperating the same.

BACKGROUND

HVAC systems should be capable of operating efficiently under a widerange of outdoor temperatures. Current HVAC systems control outdoor(condenser) fan and indoor blower speed based on the cooling required tobe provided by the system. In many such systems, outdoor fan speed iscontrolled such that refrigerant pressure remains within a desiredrange. Excessive pressure risks refrigerant leakage, and inadequatepressure risks compressor damage or failure. Subject to maintainingpressure within a desired range, the system as a whole is thencontrolled to operate as efficiently as possible. Some HVAC systemsemploy multistage compressors and multiple outdoor fans to increaseoperating efficiency.

SUMMARY

One aspect provides an HVAC controller. In one embodiment, the HVACcontroller includes: (1) a processor couplable to at least tworefrigerant pressure sensors via separate data paths to receive inputsignals therefrom and further couplable to a compressor stage and acondenser fan to provide output signals thereto, and (2) memory coupledto the processor and storing a software program having programinstructions capable of causing the processor to command the compressorstage or the condenser fan to turn on irrespective of a state of aninput signal generated by either of the at least two refrigerantpressure sensors, and generate an error message at least partiallydepending upon whether or not a high pressure shutdown occurs after theprocessor commands the compressor stage or the fan to turn on.

Another aspect provides a method of operating an HVAC system. In oneembodiment, the method includes: (1) commanding a compressor stage or anassociated condenser fan to turn on irrespective of a state of an inputsignal generated by at least two refrigerant pressure sensors associatedwith the condenser fan, and (2) generating an error message at leastpartially depending upon whether or not a high pressure shutdown occursafter the commanding.

Another aspect provides an HVAC system. In one embodiment, the HVACsystem includes: (1) an outdoor unit, including: (1 a) at least twocompressor stages, (1 b) at least two corresponding condenser fans, (1c) at least two corresponding refrigerant pressure sensors, (1 d) atleast one condenser coil and (1 e) an outside air temperature sensor,(2) an indoor unit, including: (2 a) at least one evaporator coil, (2 b)at least one indoor blower and (2 c) at least one expansion valve and(3) an HVAC controller, including: (3 a) a processor couplable to atleast two refrigerant pressure sensors via separate data paths toreceive input signals therefrom and further couplable to a compressorstage and a condenser fan to provide output signals thereto, and (3 b)memory coupled to the processor and storing a software program havingprogram instructions capable of causing the processor to command thecompressor stage or the condenser fan to turn on irrespective of a stateof an input signal generated by either of the at least two refrigerantpressure sensors, and generate an error message at least partiallydepending upon whether or not a high pressure shutdown occurs after theprocessor commands the compressor stage or the fan to turn on.

BRIEF DESCRIPTION

Reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a block diagram of one embodiment of an HVAC system includingone embodiment of an outdoor fan and indoor blower controllerconstructed according to the principles of the invention;

FIG. 2 is a block diagram of one embodiment of the controller of FIG. 1;

FIGS. 3A and 3B are flow diagrams of one embodiment of a method ofoperating an outdoor fan of an HVAC system carried out according to theprinciples of the invention; and

FIG. 4 is a flow diagram of one embodiment of a method of operating anoutdoor fan of an HVAC system carried out according to the principles ofthe invention

DETAILED DESCRIPTION

FIG. 1 is a block diagram of one embodiment of an HVAC system 100including one embodiment of an outdoor fan and indoor blower controllerconstructed according to the principles of the invention. The HVACsystem 100 includes an outdoor unit 110, which may be a rooftop unit,and an indoor unit 120. The outdoor unit 110 and the indoor unit 120 arerepresented as being separate, but in fact may be housed in a commonenclosure.

The illustrated embodiment of the outdoor unit 110 includes one or morecompressors each having one or more stages 111. One or more condenserfans 112 are associated with one or more condenser coils 113 to move airacross the one or more condenser coils 113. An outside air temperaturesensor 114 is situated in or on the outdoor unit 110 to detect anambient outdoor air temperature, and one or more refrigerant pressuresensors 115 are situated in or on the outdoor unit to detect refrigerantpressure in the one or more condenser coils 113. In the illustratedembodiment, at least one refrigerant pressure sensor, a low ambientpressure switch, is associated with each condenser coil and isconfigured to change switch state (open or close) as a function of thepressure of refrigerant in its associated coil relative to apre-established pressure at a lower end of an acceptable pressure range.In another embodiment, a high ambient pressure switch is also associatedwith each condenser coil and is configured to change switch state as afunction of the pressure of refrigerant in its associated coil relativeto a pre-established pressure at a higher end of an acceptable pressurerange.

The illustrated embodiment of the indoor unit 120 includes one or moreevaporator coils 121. One or more blowers 122, sometimes known as indoorblowers, are associated with the one or more evaporator coils 121 tomove air across the one or more evaporator coils 121. One or moreexpansion valves 123 are coupled to one or more correspondingrefrigerant conduits 124. The one or more refrigerant conduits 124couple the one or more stages 111 of the one or more compressors, theone or more condenser coils 113, the one or more expansion valves 123and the one or more evaporator coils 121 to form a loop within which arefrigerant (e.g., a hydrofluorocarbon fluid) is repeatedly compressed,cooled, decompressed and warmed to effect air conditioning. In oneembodiment, the indoor unit 120 includes one or more heater coils (notshown) associated with the one or more blowers 122 to effect heating. Inanother embodiment, the one or more blowers 122 may be activatedseparately to effect ventilation.

As stated above, the illustrated embodiment of the system 100 furtherincludes an outdoor fan and indoor blower controller 130. Theillustrated embodiment of the controller 130 is configured to receiveinput signals from, perhaps among other things, the outside airtemperature sensor 114 and the one or more refrigerant pressure sensors115 and generate output signals to control, perhaps among other things,the one or more condenser fans 112 and the one or more blowers 122. Auser interface (not shown), perhaps including an indoor temperaturesensor, is coupled to the controller 130 and configured to allow a userto select a setpoint indoor temperature and perhaps a system operationalmode (i.e., air conditioning, heating or ventilation). Those skilled inthe pertinent art are familiar with the manner in which HVAC systems,such as the HVAC system 100 of FIG. 1, may be controlled by a user.

FIG. 2 is a block diagram of one embodiment of the outdoor fan andindoor blower controller 130 of FIG. 1. In the embodiment of FIG. 2, thecontroller 130 takes the form of a general purpose microcontroller andcontains a processor 210 configured to execute software (e.g., firmware)instructions, a volatile memory 220 coupled to the processor 210 andconfigured to store software instructions, data or both softwareinstructions and data and nonvolatile memory 230 coupled to theprocessor 210 and configured to store software instructions, data orboth software instructions and data. In the embodiment of FIG. 2, thenonvolatile memory 230 stores the software instructions and persistentdata (e.g., factory settings and messages) that enable the operation ofthe controller 130, and the volatile memory 220 stores data that thecontroller 130 collects during its operation and stores temporarily forinternal use or external recall (e.g., scratchpad data and operationallogs).

As FIG. 2 shows, an outside air temperature sensor 114 and first andsecond refrigerant pressure sensors 115-1, 115-2 are coupled to theprocessor 210 to provide input signals thereto. Likewise, the processor210 is coupled to first and second compressor stages 111-1, 111-2 andcorresponding first and second condenser fans 112-1, 112-2. Thus thespecific embodiment of the controller 130 illustrated in FIG. 2 isconfigured for use in an HVAC system that has two compressor stages, twocondenser coils and two corresponding condenser fans. Of course, asstated above, other embodiments of the controller 130 accommodate othernumbers of compressor stages, condenser coils and condenser fans.

It should be noted that each of the sensors 114, 115-1, 115-2 has aseparate data path to the processor 210, and that the processor 210 hasa separate data path to each of the stages and fans 111-1, 111-2, 112-1,112-2. The provision of the separate data path may be colloquiallyreferred to as “home running.” The separate data paths may be separatewireline buses or wireless channels or time-divided or code-dividedallocations of a shared wireline bus or wireless channel. The object ofthe separate data paths is that each of the sensors 114, 115-1, 115-2can transmit its input signal separately to the processor 210, and theprocessor can transmit its output signals separately to each of thestages and fans 111-1, 111-2, 112-1, 112-2. Thus the output of each ofthe sensors 114, 115-1, 115-2 can be separately sensed, and each of thestages and fans 111-1, 111-2, 112-1, 112-2 can be separately controlled.

As stated above, many HVAC systems control outdoor fan speed such thatrefrigerant pressure remains within a desired range. In systems havingonly a single compressor stage, a pressure sensor on the condenser coildirectly controls the one or more fans; no effort is made to control thecondenser fans based on outside air temperature. In systems havingmultiple compressor stages, some applications benefit from controllingthe condenser fans based on both condenser coil pressure and outside airtemperature. Unfortunately, the controller's hardware interlock preventsthis from being achieved directly. Instead, it is achieved by bypassingthe interlock and connecting multiple pressure sensors in parallel.Besides being cumbersome, the parallel-coupled sensors cannot beseparately detected or diagnosed. As a result, a single faulty sensorcan needlessly impair the operation of the HVAC system as a whole,either by wasting energy by causing one or more fans to operate whenthey need not or by risking harm to one or more compressors bypreventing the one or more fans from operating when they should. Thecontroller 130 of FIG. 2 does not have a hardware interlock and thusaccommodates home running of the multiple pressure sensors. As a result,the controller 130 avoids the need to couple pressure sensors inparallel and allows sensor-specific diagnostics.

In various embodiments, the controller 130 allows multiple fans to becontrolled based on pressure, temperature, or both pressure andtemperature at different setpoints, even for a single compressor system,without additional control apparatus other than that needed to power thefan. In various other embodiments, the controller 130 can control morethan one fan based on different temperature setpoints or based on theinput signals produced by any of the pressure sensors. As a result, thecontroller 130 can detect the status of the different pressure sensors,act according to a pre-programmed sequence of operation, and determinebased on the different conditions whether a given pressure sensor isgood or faulty. Once a faulty sensor is identified, various embodimentsof the controller 130 can provide error messages (e.g., codes orphrases) for service, including component repair or replacement, whilecontinuing to run the one or more fans without the faulty sensor. Theability to continue to run the HVAC system even when a pressure sensoris faulty prevents potential damage that may result from a faultypressure sensor and increases the overall reliability of the HVACsystem.

FIGS. 3A and 3B are flow diagrams of one embodiment of a method ofoperating an outdoor fan of an HVAC system carried out according to theprinciples of the invention. The method of FIG. 3A begins in a startstep 305, when conditions are such that air conditioning is desirable.In a step 310, the one or more refrigerant pressure sensors (e.g., oneor more low and/or high ambient pressure switches) are read. In a step315, the outside air temperature sensor is also read. Based on thestates of the one or more refrigerant pressure sensors and the outsideair temperature sensor, one or more stages of the compressor arecontrolled (e.g., turned on or off) in a step 320. Also, based on thestates of the one or more refrigerant pressure sensors and the outsideair temperature sensor, one or more condenser fans are controlled (e.g.,turned on or off) in a step 325. For example, if an outdoor airtemperature is 85° F. and a desired indoor setpoint temperature is 72°F., the size of the HVAC system may have been chosen such that only asingle stage of a two-stage compressor is needed to maintain the desiredsetpoint temperature at that given outdoor air temperature. Accordingly,the controller produces an output signal that commands the single stageto begin operation. As a result, refrigerant pressure downstream of thecompressor stage increases, causing the associated low ambient pressureswitch to change state and generate a corresponding input signal to thecontroller. This input signal, perhaps in conjunction with other inputsignals or parameters such as time, in turn causes the controller toturn on an associated fan to reduce the rate of increase of therefrigerant pressure. The method ends in an end step 330.

However, turning now to FIG. 3B, it will now be assumed that either alow ambient pressure switch or a condenser fan has failed. The method ofFIG. 3B presents one embodiment of a method by which diagnostics may beperformed with respect to the HVAC system and begins in a start step340, when it is desired to activate air conditioning. Accordingly, thecontroller turns on at least one compressor stage in a step 345.Assuming a high ambient pressure switch located downstream of thecompressor stage is coupled to the controller, the output signal fromthat switch determines the outcome of a decisional step 350. The purposeof the high ambient pressure switch is to protect the HVAC systemagainst the harm that may result from excessive refrigerant pressure. Afailure of a fan to operate to cool its associated condenser coil isoften the cause of excessive refrigerant pressure. If the output signalindicates that refrigerant pressure remains in the acceptable range,normal operation ensues in a step 355, and the method ends in a step360.

On the other hand, if excessive refrigerant pressure causes the highambient pressure switch to change state, it cannot directly bedetermined whether a faulty low pressure ambient switch failed to changestate to activate the condenser fan, or whether the low ambient pressureswitch did close, but the fan was unable to respond, perhaps due to afault in wiring leading to the fan, an actuator providing power to thefan, or the fan itself. Irrespectively, if the output signal of a highambient pressure switch causes the controller to respond with a highpressure shutdown of the HVAC system, a preprogrammed delay ensues,after which the controller generates an output signal to command the atleast one compressor stage to turn on again in a step 365. Thecontroller then generates an output signal to command the associated atleast one condenser fan turn on again in a step 370. If a high pressureshutdown does not again ensue in a decisional step 375, the controllercan then assume that the associated at least one fan did turn on andthat a faulty low ambient pressure switch likely prevented the at leastone fan from turning on previously. The controller then generates anappropriate error message indicating that a low ambient pressure switchis faulty in a step 380. The controller then causes the HVAC system tooperate under a fault condition (namely the faulty low ambient pressureswitch) in a step 385, whereupon the method ends in the end step 355. Invarious embodiments, the controller may also alert the customer that thesystem is operating under a fault condition if the controller is part ofa network, e.g., a Building Automation System (BAS).

If, on the other hand, a high pressure shutdown does again ensue in thedecisional step 375, the controller can then assume that the at leastone fan did not turn on as commanded and that the at least one fan orwiring leading to it is faulty. The controller then generates anappropriate error message indicating that one or more condenser fans arenot operating in the step 380. The controller can then attempt continuedoperation of the HVAC system under fault condition. As above, thecontroller may also alert the customer that the system is operatingunder a fault condition if the controller is part of a network. In oneembodiment, the controller determines whether alternative compressors orstages associated with operable condenser fans may be turned on. Inanother embodiment, the controller operates the fewer remaining operablefans at a higher speed if the system has a variable speed fan orElectronic Conmutated Motor (ECM). In yet another embodiment, thecontroller turns on one or more fans that had been turned off. In stillanother embodiment, the controller determines that other fans are notavailable and commands the HVAC system to shut down pending repair. In amore specific embodiment, the controller broadcasts an alarm signalthrough a network to inform the customer of a shutdown condition so thesystem can be repaired.

Various embodiments of the controller can perform other operations thatallow the HVAC system to operate under conditions in which outside airtemperatures are exceptionally cold, for example, less than 55° F.Conventional HVAC systems have trouble operating under suchexceptionally cold conditions, because while their condenser fans shouldoperate to avoid excessive refrigerant pressure in the condenser coils,operation of the fans in such low outside air temperatures can over-coolthe condenser coils, causing inadequate refrigerant pressure. As statedabove, operation either above or below an acceptable refrigerantpressure range can harm the HVAC system. Introduced herein are variousembodiments of an HVAC controller and method for accommodating HVACsystem operation under relatively low outside air temperatures. FIG. 4is a flow diagram of one embodiment of a method of operating an outdoorfan of an HVAC system carried out according to the principles of theinvention. The method begins in a step 410. In a step 420, refrigerantpressure is detected.

In one embodiment, refrigerant pressure is detected by determining if itis within or without an acceptable range. This can be performed with lowand high ambient pressure switches. Depending upon the states of the twopressure switches, it can be determined whether the refrigerant is: (1)below a lower pressure threshold, (2) above the lower threshold butbelow an upper pressure threshold, or (3) above the upper threshold.

In a decisional step 430, if the refrigerant temperature is above theupper threshold (indicating a refrigerant pressure above the acceptablerange), the outcome of the decisional step 430 is YES, and thecontroller commands a condenser fan to increase its speed to the nexthigher speed in a step 440. For example, if the condenser fan is runningat a low-low speed, the controller commands the condenser fan toincrease its speed to a low speed. If the condenser fan is running at alow speed, the controller commands the condenser fan to increase itsspeed to a high speed. The refrigerant pressure is detected again at alater time in the step 420. For purposes of this invention, a low-lowspeed is a speed that is lower than the speed at which a fan or blowernormally runs when the HVAC system is first-stage cooling. In theembodiment of FIG. 4, an example low-low speed may be about 300 RPM, alow speed may be about 600 RPM, and a high speed may be about 900 RPM.

If the outcome of the decisional step 430 is NO, in a decisional step450 it is determined if the refrigerant temperature is above the lowerthreshold (indicating a refrigerant pressure within the acceptablerange), the outcome of the decisional step is YES, and the controllerdoes not command the condenser fan speed to change. The refrigerantpressure is detected again at a later time in the step 420.

If the outcome of the decisional step 450 is NO (indicating arefrigerant pressure below the acceptable range), it is determined in adecisional step 460 if the condenser fan is running at a low-low speed.If the outcome of the decisional step 460 is YES, the method proceeds tothe step 470 in which the controller calls for the condenser fan tocontinue to operate at the low-low speed until the lower threshold isreached, at which time the controller calls for the condenser fan toturn off. The condenser fan then remains off until the upper thresholdis reached, at which time the controller calls for the fan to turn backon at the low-low speed. The refrigerant pressure is detected again at alater time in the step 420.

If the outcome of the decisional step 460 is NO, and the controllercommands a condenser fan to decrease its speed to the next lower speedin a step 480. For example, if the condenser fan is running at a highspeed, the controller commands the condenser fan to decrease its speedto a low speed. If the condenser fan is running at a low speed, thecontroller commands the condenser fan to decrease its speed to a low-lowspeed.

In certain embodiments, the controller can control an indoor blower tooperate at a low-low speed to improve ventilation or for other purposeswhen the compressor is turned off. In still other embodiments, thecontroller may command a damper to open to allow outdoor air to flow tothe blower, perhaps across a filter to reduce particulate matterbeforehand. This ostensibly lowers the temperature of the indoor air theblower is circulating.

Those skilled in the art to which this application relates willappreciate that other and further additions, deletions, substitutionsand modifications may be made to the described embodiments.

What is claimed is:
 1. An HVAC controller, comprising: a processorcouplable to at least two refrigerant pressure sensors via separate datapaths to receive input signals therefrom and further couplable to acompressor stage and a condenser fan to provide output signals thereto;and memory coupled to said processor and storing a software programhaving program instructions capable of causing said processor to commandsaid compressor stage or said condenser fan to turn on irrespective of astate of an input signal generated by either of said at least tworefrigerant pressure sensors and generate an error message at leastpartially depending upon whether or not a high pressure shutdown occursafter said processor commands said compressor stage or said fan to turnon.
 2. The HVAC controller as recited in claim 1 wherein said at leasttwo refrigerant pressure sensors comprise at least two low ambientpressure switches.
 3. The HVAC controller as recited in claim 1 whereinsaid program instructions are capable of causing said processor tocommand said compressor stage and said condenser fan to turn onirrespective of a state of an input signal generated by either of saidat least two refrigerant pressure sensors and generate an error messageat least partially depending upon whether or not a high pressureshutdown occurs after said processor commands said compressor stage andsaid fan to turn on.
 4. The HVAC controller as recited in claim 3wherein said software program is further capable of causing saidprocessor to command said high pressure shutdown and causing saidprocessor to command said compressor stage and said fan to turn on afteran initial high pressure shutdown occurs.
 5. The HVAC controller asrecited in claim 1 wherein said processor is further couplable to anoutside air temperature sensor via a separate data path to receive aninput signal therefrom and said software program is further capable ofcausing said processor to generate said output signals at leastpartially based on said input signal from said outside air temperaturesensor.
 6. The HVAC controller as recited in claim 1 wherein saidsoftware program is further capable of causing said processor to commandsaid compressor stage to turn on and said condenser fan to run at alow-low speed based on a refrigerant pressure.
 7. The HVAC controller asrecited in claim 1 wherein said processor is further couplable to anindoor blower to provide an output signal thereto and said softwareprogram is further capable of causing said processor to command saidcompressor stage to turn off, said condenser fan to run and said indoorblower to run at a low-low speed.
 8. A method of operating an HVACsystem, comprising: commanding a compressor stage or an associatedcondenser fan to turn on irrespective of a state of an input signalgenerated by at least two refrigerant pressure sensors associated withsaid condenser fan; and generating an error message at least partiallydepending upon whether or not a high pressure shutdown occurs after saidcommanding.
 9. The method as recited in claim 8 wherein said at leasttwo refrigerant pressure sensors comprises at least two low ambientpressure switches.
 10. The method as recited in claim 8 whereincommanding includes commanding said compressor stage and said associatedcondenser fan to turn on irrespective of said state of said input signalgenerated by at least two refrigerant pressure sensors associated withsaid condenser fan.
 11. The method as recited in claim 8 furthercomprising: commanding said high pressure shutdown; and commanding saidcompressor stage and said fan to turn on after an initial high pressureshutdown occurs.
 12. The method as recited in claim 8 furthercomprising: receiving an input signal from an outside air temperaturesensor; and generating output signals at least partially based on saidinput signal from said outside air temperature sensor.
 13. The method asrecited in claim 8 further comprising commanding said compressor stageto turn on and said condenser fan to run at a low-low speed based on arefrigerant pressure.
 14. The method as recited in claim 8 furthercomprising commanding said compressor stage to turn off, said condenserfan to run and said indoor blower to run at a low-low speed.
 15. An HVACsystem, comprising: an outdoor unit, including: at least two compressorstages, at least two corresponding condenser fans, at least twocorresponding refrigerant pressure sensors, at least one condenser coil,and an outside air temperature sensor; an indoor unit, including: atleast one evaporator coil, at least one indoor blower, and at least oneexpansion valve; and an HVAC controller, including: a processorcouplable to said at least two refrigerant pressure sensors via separatedata paths to receive input signals therefrom and further couplable tosaid at least two compressor stages and said at least two condenser fansto provide output signals thereto, and memory coupled to said processorand storing a software program having instructions capable of causingsaid processor to command one of said at least two compressor stages ora corresponding one of said at least two condenser fans to turn onirrespective of a state of an input signal generated by either of saidat least two refrigerant pressure sensors and generating an errormessage at least partially depending upon whether or not a high pressureshutdown occurs after said processor commands said one of said at leasttwo compressor stages or said corresponding one of said at least twocondenser fans to turn on.
 16. The system as recited in claim 15 whereinsaid at least two refrigerant pressure sensors comprise at least two lowambient pressure switches.
 17. The system as recited in claim 15 whereinsaid software program has instructions capable of causing said processorto command one of said at least two compressor stages and acorresponding one of said at least two condenser fans to turn onirrespective of a state of an input signal generated by either of saidat least two refrigerant pressure sensors and generating an errormessage at least partially depending upon whether or not a high pressureshutdown occurs after said processor commands said one of said at leasttwo compressor stages and said corresponding one of said at least twocondenser fans to turn on.
 18. The system as recited in claim 17 whereinsaid software program is further capable of causing said processor tocommand said high pressure shutdown and causing said processor tocommand said one of said at least two compressor stages and saidcorresponding one of said at least two fans to turn on after an initialhigh pressure shutdown occurs.
 19. The system as recited in claim 17wherein software program is further capable of causing said processor togenerate said output signals at least partially based on said inputsignal from said outside air temperature sensor.
 20. The system asrecited in claim 17 wherein said software program is further capable ofcausing said processor to command said one of said at least twocompressor stages to turn on and said corresponding one of said at leasttwo condenser fans to run at a low-low speed based on a refrigerantpressure.
 21. The system as recited in claim 17 wherein said softwareprogram is further capable of causing said processor to command said oneof said at least two compressor stages to turn off, said one of said atleast two condenser fans to run and one of said at least two indoorblowers to run at a low-low speed.